1. Field of the Invention
The present invention relates to a polypropylene resin foamed bead and a molded article of the foamed beads, and, particularly, to a polypropylene resin foamed bead which can form a foamed bead molded article superior in appearance, heat resistance and mechanical properties at a molding temperature lower than the molding temperature of conventional polypropylene resin foamed beads and to a foamed bead molded article obtained by molding the polypropylene resin foamed beads.
2. Description of the Prior Art
Polypropylene resins are superior in characteristics such as mechanical strength, heat resistance, processability, incineration properties and recycling characteristics, and their fields of applications are growing widely. Polypropylene resin foamed bead molded article obtained by molding of polypropylene resin foamed beads can further shock-absorbing property, insulation property and light-weight characteristics without losing the above excellent natures of polypropylene resins and are being widely used for packaging materials, construction materials and impact absorption materials for vehicles.
Polypropylene resin foamed bead molded articles obtained by filling polypropylene resin foamed beads in a mold and by carrying out expanding and fuse-bonding operations using heating steam and the like are superior in heat resistance, chemical resistance, toughness and compression strain recovery characteristics to polystyrene resin foamed molded articles to be used in similar applications. On the contrary, heating at a higher temperature, that is, heating using steam having a higher saturated vapor pressure than in the case of the molding of polystyrene resin foamed beads is required to secondary expansion the polypropylene resin foamed beads (hereinafter referred to as secondary expanding) and also to carryout the fusion among beads in molding. For this, it is necessary to prepare a mold which meets application requirements for high-pressure resistance and a molding machine specified for high press pressure, which increases the energy cost to be used.
In order to solve such problems, Patent Reference 1, for example, proposes that the surface of polypropylene resin foamed beads is coated with a different resin having a low-melting point. However, the equipment used for this coating is complicated and also, the process is complicated. Also, though the fusion ability among famed beads during molding is improved, the secondary expanding characteristics of foamed beads are insufficient. Therefore, the molded article has unsatisfactory appearance and there is a room for an improvement in appearance. In order to solve these problems, it is necessary to provide an inner pressure applying process for raising the inner pressure in the foamed beads, to enable molding under a high compressive ratio or to raise the molding pressure, for the purpose of raising the secondary expanding ability of the foam particles.
There is a method using a polypropylene resin having a relatively low melting point as its base resin in order to drop the molding temperature of foamed beads in molding. As shown in Patent Reference 2, polypropylene resin foamed beads are known which are produced by using, as its base resin, a polypropylene resin polymerized using a metallocene type polymerization catalyst. As the polypropylene resin polymerized by using this metallocene type polymerization catalyst, a resin having a lower melting point than a usual polymer produced using a Ziegler-Natta type catalyst is easily obtained. However, there is much room for improvements in the effect of dropping the saturated vapor pressure of the steam required as the heating medium in the molding, the appearance of the foamed bead molded article obtained by the molding and molding stability such as the mutual fusibility of the foamed beads.
Also, as shown in Patent Reference 3, non-crosslinked foamed beads containing, as the base resin, a modified polypropylene resin is known which is obtained by impregnating a polypropylene resin with a vinyl monomer, followed by graft polymerization and in which the ratio of “content of a polypropylene resin”/“content of a polymer of a vinyl monomer” is 97 to 65% by weight/3 to 35% by weight. The heat resistance of the foamed beads obtained by impregnating the polypropylene resin particles with a vinyl monomer, followed by graft polymerization tends to be dependent on the transition of state, such as melting point or glass transition temperature, specific to a polymer. It is therefore considered to be able to drop the saturated vapor pressure of steam in molding by selecting a polypropylene resin having a low melting point. However, this poses a problem in the point of keeping heat resistance.
Patent Reference 4 proposes a method in which two types of polypropylene resins differing in melting point are mixed. In this Patent Reference, for example, a polypropylene resin foamed bead is reported which has an expansion ratio of 10 to 50 and contains, as its base resin, a polypropylene resin which is obtained by mixing two types of polypropylene resins differing in melting point by 15° C. or more and 30° C. or less and has a melt index of 3 g/10 min or more and 20 g/10 min or less. However, heating temperature exceeding 140° C. is still required for the molding of the proposed polypropylene resin foamed beads and steam having a high saturated vapor pressure is used.
Patent Reference 5 discloses that a polypropylene resin foamed bead molded article which has a specified endothermic peak and an endothermic temperature and has the specified relation between the endothermic peak temperature and endothermic calorific value is a high-density foam which has a uniform cell diameter and good appearance and has low heat conductivity. However, the foamed beads and molded article described in Patent Reference 5 are respectively only a type which has one endothermic peak at the high-temperature side of the main endothermic peak on the first DSC curve obtained by measuring heat flux differential scanning calorific value which will be explained later in the present invention, and it is difficult to obtain a foamed bead molded article having stable and excellent properties by molding at a low molding temperature.    Patent Reference 1: JP-A No. 2000-894    Patent Reference 2: JP-A No. 6-240041    Patent Reference 3: JP-A No. 10-292064    Patent Reference 4: JP-A No. 2006-96805    Patent Reference 5: JP-A No. 2001-341151